602
Efficiency of Steam.
Commencing at o, Fig. 611, draw the co-ordinates ov, ox, for
pressure and distance respectively. Measure 26-36 ft. at OA, and
2116-8 Ibs. at OB; the rectangle AB then shews external work.
Make OD and D E 12" 36 and 2'i times o B respectively; the area
o F is the internal work during evaporation, and D G shews the
work required to raise the water's temperature from 60° to 212°.
Rectangle AB represents the only useful effect, the rest being
expended on internal changes, and the
external work
^~. . ,. ,
Efficiency of the steam
- - -
5,799
------- r .- = *' ^
total work 863,096
= '0646
— -
Let us next examine the case of steam at 160 Ibs. pressure
(above atmosphere), as in triple-expansion engines.
i Ib. of steam at 174*7 Ibs. per sq. in. absolute has a specific
volume of .......... 2*5 cub. ft.
Load on piston = 174*7 x 144 ..... = 25,156 Ibs.
(1) External work = 25,156 x 2-5 . . . . = 62,890 ft. Ibs.
Temperature of steam ...,...«= 370° F.
Latent heat = (966 - "7(370 - 212)} x 772 = 660,369 ft. Ibs.
(2) Internal work = (660,369 - 62,890) . . = 597, 479 ft. Ibs.
(3) Heat to raise water's temperature
= (370-60)772 = 239,320 ft. Ibs.
And total work = (i) -H (2) + (3) . . . = 899,689 ft. Ibs.
Efficiency of steam
external work _^ 62,890
total work " 899,189
-07.
Which proves that high pressure steam, weight for weight and
without expansion^ is not more economical than low pressure
steam.
Specific Heats of a Gas.—As with solids and liquids
these are the quantity of heat required to raise the temperature
of i Ib. weight through one degree F. But there are two methods
of raising the temperature, the specific heat being a different
quantity for each case. Assuming the gas enclosed in a cylinder
and covered with a loose piston, we may, while supplying heat,
(i) allow the piston to rise freely, or (2) fix it immovably. In
™*~ are heating at constant pressure, and in (2) at constant